The present invention relates to the field of pipe extrusion dies, and in particular, to a pipe extrusion die that can be used to form a pipe with multiple layers of varying thickness and temperature profiles.
The ability to provide an extrusion die that allows for the efficient manufacture of a pipe with multiple seamless layers has been a constant goal of extrusion die manufacturers.
U.S. Pat. No. 5,346,384 to Hegler and U.S. Pat. No. 4,789,327 to Chan provide two examples of extrusion dies used to produce a double-walled thermoplastic pipe. U.S. Pat. No. 4,789,327 to Chan discloses the general structure of the extrusion die, which includes i) an extrusion head having a central bore and a lateral opening for receiving an extrudate; (ii) a nozzle operably connected to the extrusion head, (iii) two hollow mandrels carrying die lips, (iv) two layer-forming channels; and, (v) a cooling mandrel. The layer-forming channels have an entrance passageway that derives from the extrusion head and a discharge end formed by the die gap created by the outer and inner die lip.
More particularly, U.S. Pat. No. 4,789,327 to Chan uses two hollow mandrels, one placed inside the other. The space between the nozzle and the hollow mandrel of the first die assembly comprises the first layer-forming channel and the space between the hollow mandrel of the first die assembly and the hollow mandrel of the second die assembly comprises the second layer-forming channel. Adjustment of the die gap occurs by threaded die lip adjusters. Accordingly, the outer nozzle has to be moved longitudinally in order to adjust the die gap.
The hollow mandrel of the first die assembly is held inside the hollow mandrel of the second die assembly by a spider. The use of a spider to hold the mandrels, one inside the other, results in several disadvantages. In particular, marks appear on the finished pipe, there is an increase in back pressure due to a reduced flow of plastic through the spider legs, heat is lost through the spider section, and leakage occurs at the spider junctions.
U.S. Pat. No. 4,808,098 to Chan provides an extrusion die with the same features as U.S. Pat. No. 4,789,327, but further includes a vacuum cooling mandrel that uses multiple communicating cooling channels along with a few vacuum ports. Distribution of the cooling vacuum therefore occurs, only at a few selected sites, i.e., the location of the vacuum ports.
None of the prior art references, therefore, provide for a pipe extrusion die capable of forming a pipe with more than two layers made from different materials, with varied thickness and temperature profiles.
In one aspect of the present invention, the disadvantages of the prior art are overcome by providing a pipe extrusion die having a space separating the layer-forming channels. The structure of the pipe extrusion die with this space allows for pipes of more than two layers to be formed that may consist of different materials having different temperature profiles. Having a space between the layer-forming channels allows the added advantage that the individual die assemblies can be moved longitudinally prior to and during manufacture.
In another aspect of the present invention, there is also provided an extrusion die that allows for adjustment of the die lips from a location near the extrusion head. Adjustment of the die lips at a location near the extrusion head allows for adjustment of individual die layers and for die layers to be modified during manufacture of the pipe.
In a further aspect of the present invention, there is provided an improved vacuum cooling mandrel that uses a single cooling channel with multiple vacuum ports to provide more control over the structure and appearance of the pipe that is formed.
According to one aspect of the invention, therefore, there is provided an extrusion die for use in an apparatus to produce multi-layered pipes, the extrusion die comprising: (a) a first and second die assembly, each die assembly comprising: (i) an extrusion head having a central bore and a lateral opening for receiving an extrudate; (ii) a nozzle, operably connected to the extrusion head, the nozzle having an outer die lip at a free end of the nozzle; (iii) a hollow mandrel coaxially located in the central bore, (iv) an inner mandrel coaxially located in the hollow mandrel and in the nozzle, the inner mandrel having an inner die lip at a free end of the inner mandrel; the inner die lip and outer die lip defining a die gap; and, (v) the nozzle and inner mandrel defining a layer-forming channel in fluid communication with the lateral opening and the die gap; wherein a central portion of the nozzle of the second die assembly is co-axially located within the inner mandrel of the first die assembly such that the nozzle of the second die assembly and the inner mandrel of the first die assembly define an air space.
According to another aspect of the invention, there is provided an extrusion die for use in an apparatus to produce multi-layered pipes, the extrusion die comprising: (a) a first and second die assembly, each die assembly comprising: (i) an extrusion head having a central bore and a lateral opening for receiving an extrudate; (ii) a nozzle operably connected to the extrusion head, the nozzle having an outer die lip at a free end of the nozzle; (iii) a hollow mandrel coaxially located in the central bore; (iv) an inner mandrel coaxially located in the hollow mandrel and in the nozzle, the inner mandrel having an inner die lip at a free end of the inner mandrel; the inner and outer die lips defining a die gap; and, (v) the nozzle and inner mandrel defining a layer-forming channel in fluid communication with the lateral opening and the die gap; wherein a central portion of the nozzle of the second die assembly is co-axially located within the inner mandrel of the first die assembly.
According to yet another aspect of the invention, there is provided a vacuum cooling mandrel for an extrusion die, said vacuum cooling mandrel comprising a single cooling channel and multiple vacuum ports, said multiple vacuum ports disposed along an outer surface of the cooling mandrel.
According to yet another aspect of the invention, there is provided a method for preparing a multi-layered pipe using the extrusion die, wherein said method includes the steps of: (a) introducing under pressure, a first extrudate into the lateral opening of the first die assembly and introducing a second extrudate into the lateral opening of the second die assembly; (b) passing said first and second extrudates through said first and second layer-forming channels; and, (c) receiving said first extrudate from the die gap of the first die assembly and receiving the second extrudate from the die gap of the second die assembly.
In another aspect of the invention, the method further includes the step of adjusting the longitudinal distance between the die gap of the first die assembly and the die gap of the second die assembly by moving the second die assembly longitudinally within the inner mandrel of the first die assembly. The method may also include the steps of introducing air pressure into the air space; altering the size of the die gap by using means near the extrusion head for moving the inner die lip longitudinally; shaping the pipe using the vacuum cooling mandrel; and, introducing a vacuum through the vacuum port holes.
Various other advantages will be apparent to the person skilled in the art from the following description of the present invention when read in conjunction with the accompanying drawings.